<template>
    <div class="list">
        <ul>
            <li 
            class="item" 
            v-for="(item,key) in letters" 
            :key="key"
            :ref="item"
            @click="handleLetterClick"
            @touchstart="handleTouchStart"
            @touchmove="handleTouchMove"
            @touchend="handleTouchEnd"
            >
                {{item}}
            </li>
        </ul>
    </div>
</template>

<script>
    export default {
        name:'CityAlphabet',
        props:{
            cities:Object
        },
        data(){
            return{
                touchStatus:false,
                startY:0,
                timer:null
            }
        },
        updated(){
            this.startY= this.$refs['A'][0].offsetTop;
        },
        computed:{
            letters(){
                const letters=[];
                for(let i in this.cities){
                    letters.push(i);
                }
                return letters;
            }
        },
        methods:{
            handleLetterClick(e){
                this.$emit('change',e.target.innerText);
            },
            handleTouchStart(){
                this.touchStatus=true;
            },
            handleTouchMove(e){
                if(this.touchStatus){
                    if(this.timer){
                        clearTimeout(this.timer)
                    }
                    this.timer=setTimeout(() => {
                        const touchY=e.touches[0].clientY-79;
                        const index=Math.floor((touchY-this.startY)/20);
                        if(index>=0&&index<this.letters.length){
                            this.$emit('change',this.letters[index])
                        }
                    }, 16);
                }
            },
            handleTouchEnd(){
                this.touchStatus=false;
            }
        }
    }
</script>

<style scoped lang="scss">
     @import '~styles/varibles.scss';
    .list{
       display:flex;
       align-items: center;
       justify-content: center;
       position:absolute;
       top:1.58rem;
       right:0;
       bottom:0;
       width:0.4rem; 
       .item{
           line-height:0.4rem;
           color:$bgColor;
       }
    }
</style>